FISHERY BULLETIN: VOL. 70, NO. 3 



were made at various times during subsequent 

 nights. Although a given resting spot was fre- 

 quently occupied over several nights by what 

 seemed to be the same iish, this did not hold true 

 for longer than a week or so. I doubt that my 

 repeated intrusions disturbed these fishes suf- 

 ficiently to influence a change of location. Usu- 

 ally a spot occupied by a resting fish on one 

 evening was vacant when next observed on a 

 subsequent night. 



Observations on Labroides phthirophagus. — 

 Individuals of this species, like other labrids, 

 seek cover for the evening during the period 

 from immediately before to just after sunset. 

 L. phthiropkagiis, like T. duperrey, finds noctur- 

 nal shelter in crevices of rock or coral. But with 

 this species, unlike the others, individuals c?n 

 readily be recognized from one day to the next, 

 owing to their close association with well-defined 

 cleaning stations. Thus, whereas observations 

 of T. duperrey each evening involved only the 

 last active member of the species, the cover-seek- 

 ing of one particular individual L. phthirophagus 

 was timed over five diflFerent evenings (Figure 

 2) . The one instance when this fish took shelter 

 5 min after sunset was the latest that any of 

 the smaller labrids were seen active. 



L. phthirophagus cleans other fishes at its 

 station until it takes shelter. Frequently other 

 species still hover at the station in soliciting 

 fashion (see Losey, 1971) after the resident 

 cleaner has retired for the night. 



Twilight Upsurge in Activity 



Visible activity among many diurnal fishes 

 rises sharply at about the time of sunset. How- 

 ever, feeding generally is not involved; rather, 

 the upsurge stems mostly from the many species 

 on the reef that migrate from one location to an- 

 other at this time, and also to increased inter- 

 specific and intraspecific aggression, at least 

 much of which is related to territoriality. In 

 addition, some species show at this time an in- 

 creased tendency to aggregate and to swim in 

 more visible locations. 



Above, I note that prior to sunset some of the 



smaller herbivores increasingly aggregate a 

 meter or so above the reef. Beginning with this 

 event, the phenomenon gains momentum, en- 

 compassing more and larger fishes, especially 

 among the surgeonfishes and parrotfishes. By 

 5 min after sunset this activity has reached its 

 maximum level, even though most of the smaller 

 species among which it first appeared by then 

 have discontinued the activity and have settled 

 under cover. 



There seem to be two major types of aggre- 

 gations: Assemblages of one type, often of 

 mixed species, mill about 1 or 2 m above certain 

 parts of the reef; and assemblages of the other 

 type, mostly of a single species, move with seem- 

 ing purpose from one location on the reef to an- 

 other. Each of these types of assemblages is 

 treated separately below. 



The milling assemblages. — During about a 10- 

 min period, 5 to 15 min after sunset, the scene 

 on many parts of the reef is dominated by large, 

 essentially stationary, mixed-species aggrega- 

 tions that mill about in certain locations 1 or 2 m 

 above the substrate. Various surgeonfishes pre- 

 dominate, especially Zeb^^asoma flavescens (163: 

 130-195 mm), Acanthunis achilles (182: 165- 

 200 mm), A. nigroris (165: 130-180 mm), A. 

 leucopareius (190: 185-210 mm), and Naso li- 

 turatus (180: 156-220 mm). Also prominent 

 are the parrotfish Scarus sordidus (230: 205- 

 260 mm), the triggerfishes Melichthys niger 

 (165: 122-195 mm), and the filefish Cantherines 

 dimerili (200: 171-240 mm). 



The tivilight migrations. — Some members of 

 many species migrate at this time from one lo- 

 cation on the reef to another. Although the pat- 

 terns remain obscure, this activity involves 

 movements that occur consistently in certain 

 locations. Evening after evening specific routes 

 are followed, in which certain species stream 

 continuously past a given point in long drawn- 

 out processions over a period of several minutes. 



Prominent participants include members of 

 certain species, including the surgeonfish Naso 

 hexacanthus (261: 202-392 mm) and the damsel- 

 fish Chromis verater (120: 100-141 mm), that 

 feed on plankton during the day high in the 



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